Numerous components, in particular the cutting edges of drilling tools or machine tools, are formed by blocks of material of the cemented carbide type which are made up of carbide particles dispersed in a metal binder phase. These materials, which are extremely hard, and therefore resistant to wear, may also be brittle. Therefore, in order to reinforce their toughness, they are subjected to treatments which are intended to introduce into them a ductile-phase composition gradient with or without the formation of new phases, the hardness of which differs from the initial hardness of the block, which result either in blocks of which the external surface, or at least a portion of that surface, is extremely hard and the internal portion is tougher, or in blocks of which the external surface, or at least a portion thereof, is tougher and the internal portion is harder.
To do this, it is possible to manufacture blocks of non-dense cemented carbide having a porosity gradient, which blocks are produced by powder metallurgy and which are caused to be infiltrated by a binder phase in order to improve their core ductility. This method is poorly suited, in particular, to the system of the WC-Co type because it leads to the partial destruction of the carbide skeleton existing before infiltration, and therefore does not enable the desired properties for a cutting edge to be obtained.
It has also been proposed to produce cemented carbides having a composition gradient by the solid-phase natural sintering of a multi-layered component, each of the layers having a different composition. However, this method does not enable the material to be completely densified and has to be followed by an expensive hot isostatic compaction treatment. Furthermore, the preparation of the cemented carbide having a composition gradient is complex since it requires the production of a succession of basic layers which fit into one another. Finally, this process has the disadvantage of not generating a continuous composition gradient.
It has also been proposed to produce such materials by liquid-phase natural sintering, which enables a material having a completely dense composition gradient to be obtained very rapidly and in a single step. However, this process has the disadvantage of attenuating the composition gradient to a fairly great extent owing to the migration of liquid between the layers of small thickness under the combined effect of infiltration and impregnation phenomena. In addition, and against all expectation, the composition gradient remains discontinuous when the duration of maintenance in the liquid state is less than a critical duration beyond which complete homogenization of the cemented carbide is observed, but is sufficient to densify the material.
Furthermore, it has been proposed to improve the operating performance of cutting tools by depositing hard coatings of nitride, carbonitride, oxide or boride on the surface of the cemented carbide. Such methods have been described, for example, in U.S. Pat. Nos. 4,548,786 and 4,610,231 (the disclosures of which are hereby incorporated by reference). However, these methods have the disadvantage of improving only the resistance of the cemented carbide to wear by abrasion, and of doing this only over small thicknesses (a few microns).
It has also been proposed to improve both the resistance to wear of the surface and the shock resistance of cemented carbides of the WC-Co type by bringing a carbon-rich gaseous phase into contact with a dense cemented carbide sub-stoichiometric in carbon. Under the effect of temperature, the carbon of the gaseous phase diffuses in the sub-stoichiometric cemented carbide and reacts with the phase η-Co3W3C, which leads to a release of cobalt which migrates towards the external surface of the cemented carbide, that is to say, behind the carbon diffusion front. This method described in the U.S. Pat. No. 4,743,515 (the disclosure of which is hereby incorporated by reference) has the disadvantage of leading to a cobalt-rich binder phase gradient over 1 or 2 mm while preserving a brittleness of the core of the component treated.
In order to overcome the various shortcomings of the methods which have just been set forth, it has been proposed to manufacture blocks of cemented carbide having a binder phase composition gradient over distances of the order of millimeters by enriching the cemented carbides by impregnation from the outside from a liquid phase capable of penetrating (or migrating) into the cemented carbide. This impregnation phenomenon corresponds to the migration of external liquid having a composition similar to that of the solid/liquid system regarded as completely dense under the sole moving force of the migration pressure generated by a local imbalance of the volume fraction of binder phase and/or the size and morphology of the solid grains. This phenomenon concerns any system made up of (solid and liquid) condensed phases which exhibits an aptitude to adapt the shape of its solid grains by absorbing liquid, thus rendering it energetically more stable, that is to say, which exhibits an Ostwald ripening with modification of the shape of the hard particles without necessarily generating an enlargement of those particles by the phenomenon of dissolution and re-precipitation.
The use of this process to manufacture cutting edges for drilling tools or cutting tools requires the production of preliminary assemblies of a dense sintered block which is to be enriched and a compacted powder pellet of impregnation material which is to introduce the binder phase by the impregnation phenomenon, and the placing of the assembly in an oven in order to effect the appropriate heat treatment. This process has the disadvantage of requiring an impregnation material which should be dimensioned relative to the desired composition gradient, which complicates the process and generally necessitates an adjustment of the impregnation surface.